The 1.55 E crystal structure of the cytoplasmic vesibule, also referred to as the tetramerization domain (T1), of Shaker potassium channel from Aplysia reveals that four identical subunits form a centrally located water-filled pore of about 20 E in length with its narrowest diameter of about 3 E. Inside the pore near the putative cytoplasmic entryway, at least 20 water molecules are immobilized to facilitate the ion flow. These porters are positioned in a four-fold symmetry-related manner, 12 of which are mainly coordinated by polypeptide backbone atoms, implying its structural rigidity and invariability regardless of the amino acids. The membrane-facing outlet of the pore shows a conformational flexibility of the protein backbone among three different crystal forms, suggesting its role as a gating element. The majority of conserved amino acids in T1 of all known subtypes of potassium channels are found in the core of the protein. This indicates the common structural framework for the tetramer assembly.